The present invention relates to a reforming apparatus which is used for preparing a reformed gas containing hydrogen as a principal component, by steam-reforming an alcohol such as methanol and the like, a hydrocarbon such as methane, butane and the like, or a fossil fuel such as naphtha, LNG and the like, as a raw material to be reformed. More particularly, the present invention relates to a reforming apparatus capable of decreasing the concentration of carbon monoxide contained in the resultant reformed gas after steam reformation, to a level of about several tens ppm.
Conventionally, a reforming apparatus, that performs steam reformation of a raw material to be reformed and produced a reformed gas containing hydrogen as a principal component, has been known. One of applications of the reformed gas is a fuel utilizable to generate electricity in a fuel cell but, in this case, since the carbon monoxide is poisonous to electrodes of the fuel cell, it is desired that the content of carbon monoxide (CO) in the reformed gas should be removed to a level of 100 ppm or less. Therefore, CO is removed from the reformed gas by employing, after the step of steam-reforming the raw material, a step of decreasing the concentration of CO in the resultant reformed gas by water-gas-shift reaction and a step of further decreasing the concentration of CO in the resultant reformed gas by selectively oxidizing CO, as disclosed in JPA HEI 5-251,104. However, conventionally, since the three reaction steps mentioned above are performed separately in respective apparatuses, a reforming system as a whole tends to be bulky. In addition, since heat sources for providing heat of reaction are needed separately in respective reaction steps, heat loss is large. Therefore, in the conventional reforming apparatus, it has been desired to lower the heat loss and to reduce the size.
On the other hand, a prior art reforming apparatus designed to reduce the size thereof is disclosed in JPA HEI 7-126,001. Referring to this reforming apparatus, the reforming apparatus includes a reformation treating layer having a reforming reaction unit or section, a shift reaction unit and a CO oxidation unit, all arranged in series with each other along the direction of flow of gas, and a combustion gas flow path layer through which combustion gas coming from a combustion part passes, and has a structure in which the reformation treating layer and the flow path layer are alternatively positioned side by side above the combustion part. Thus, since the above three reaction units in this reforming apparatus are provided with heat from the combustion gas flow path layer, the reforming apparatus can make good use of heat in the combustion part. Also, since the reforming apparatus has the three reaction units built in one apparatus, downscaling is easy to accomplish.
However, the prior art reforming apparatus involves a problem that the temperature in each of the reaction units can not be suitably controlled. That is, it is known that catalytic reactions takes place in all of the above three reaction units and that there is a range of reactive temperature required in each of the reactions that take place respectively in the three reaction units. The range of reactive temperature of steam reforming reaction, which varies according to the kind of the raw material, for example, is about 400 to 1000xc2x0 C., preferably, 600 to 900xc2x0 C., when the raw material is a hydrocarbon such as butane, and also 250 to 400xc2x0 C. when the raw material is methanol. On the other hand, the range of reactive temperature required by the water-gas-shift reaction or the CO selective oxidation reaction does not vary so much according to the kind of the raw material, and the range of reaction temperature required by the water-gas-shift reaction is generally about 200 to 350xc2x0 C. and, preferably, 220 to 300xc2x0 C., and that required by the CO selective oxidation reaction is generally about 100 to 250xc2x0 C., preferably, 120 to 180xc2x0 C. In general, the range of reactive temperature decreases in the order of that in the reforming reaction unit, that in the shift reaction unit, that in the CO oxidation unit. Therefore, it is necessary to control the temperature in each reaction unit so as to be in the above respective range of reactive temperature. However, the reforming reaction unit and the shift reaction unit in the above prior art reforming apparatus do not separate from each other, but continued unitarily, that is, they are functionally distinguished in that the form of reaction changes from the steam reforming reaction onto the water-gas-shift reaction as the temperature of the reformed gas lowers. For this reason, even though this reforming apparatus is capable of effectively performing the steam reformation of methanol which requires a small difference between the reforming temperature and the shift reaction temperature, the steam reformation of hydrocarbons tends to exhibit a temperature diverting from the required temperature range during a transit from the reforming unit to the shift reaction unit and, therefore, a problem would arise with the hydrocarbon such as butane of which reactive temperature range during the steam reforming reaction is high. Also, since the prior art reforming apparatus has a laminated structure that the reformation treating layers and the combustion gas flow path layers alternate sidewise, the same reaction units tend to have a varying temperature depending on the position in the laminated structure, and specifically, a temperature difference between a position near to the outer periphery of the apparatus and a center position of the apparatus tends to be considerably large because the position near to the outer periphery of the apparatus is cooled by the air outside. Particularly, this varying temperature becomes problematic in the CO oxidation unit which has a narrow range of reactive temperature. Thus, when it occurs that some of the reaction units have a temperature diverting from the required temperature range, there is a fear that the hydrogen content in the resultant reformed gas lowers and the CO concentration would not be sufficiently lowered.
The present invention is capable of solving the problem inherent in the foregoing reforming apparatus according to prior art and has for its object to provide a reforming apparatus which can be downscaled by integrating the reforming reaction unit, the shift reaction unit and the CO oxidation unit together, which can make good use of heat from the heat source, and in which the temperature of each reaction unit can be favorably controlled.
A reforming apparatus according to the present invention comprises an integrated structure of three separated units including a raw material reforming unit including a heat source, that generates heat by combustion of a fuel gas, and operable to steam-reform a raw material to be reformed by directly obtaining heat for the steam reforming reaction from the heat source to produce a reformed gas containing hydrogen as a main component; a shift reaction unit for decreasing CO contained in the reformed gas, that was produced in the raw material reforming unit, by water-gas-shift reaction; and a CO oxidation unit for further degreasing CO contained in the resultant reformed gas, that was treated in the shift reaction unit, by oxidation. The raw material reforming unit, the shift reaction unit and the CO oxidation unit are arranged so that the shift reaction unit and the CO oxidation unit can be indirectly heated by heat transfer from the heat source in the raw material reforming unit.
Since this reforming apparatus includes the above three reaction units integrated together, that is, the raw material reforming unit, the shift reaction unit and the CO oxidation unit, a reformed gas removed CO can be obtained in this solo apparatus. Therefore, it is not necessary to specially provide a process for removing CO and, hence, it is capable of downscaling the system as a whole. Also, since these reaction units are independent from each other and, moreover, the steam reforming reaction requiring the highest temperature range is performed under the condition heated directly by the heat source in the raw material reforming unit, whereas, the shift reaction unit and the CO oxidation unit, that require a lower temperature range than that in the raw material reforming unit, are arranged so as to be heated indirectly, that is, by heat transfer from the heat source, each reaction unit can be controlled corresponding to the required reaction temperature range.
In the present invention, it is preferable that the raw material reforming unit, the shift reaction unit and the CO oxidation unit are concentrically arranged relative to each other with at least the CO oxidation unit placed on an outer peripheral side of the reforming apparatus. That is, the concentrical arrangement of the raw material reforming unit, the shift reaction unit and the CO oxidation unit makes it difficult to occur a bias in amount of heat transfer from the heat source and also that in amount of heat dissipation to the outside within the same reaction unit in the shift reaction unit and the CO oxidation unit. Therefore, a partial temperature distribution in the shift reaction unit and the CO oxidation unit respectively can be minimized to allow the temperature control of the shift reaction unit and the CO oxidation unit to accomplish to the required range of reactive temperature. In addition, since of the three reaction units at least the CO oxidation unit requires the temperature thereof to be controlled to the lowest range of temperature is arranged on the outer peripheral side of the reforming apparatus, heat can be easily dissipated from the CO oxidation unit, and as the result, the temperature of the CO oxidation unit can be easily controlled to the low temperature range. Also, it is easy to downscale the reforming apparatus as a whole owing to concentric arrangement.
In the present invention, where the raw material reforming unit includes a generally cylindrical combustion chamber as the heat source and a reforming reaction unit for steam-reforming the raw material to produce the reformed gas, containing hydrogen as a principal component, the reforming reaction unit may be concentrically arranged relative to the combustion chamber so as to be directly heated, and the shift reaction unit and the CO oxidation unit may be concentrically arranged relative to the combustion chamber so as to be indirectly heated. Here, the generally cylindrical shape of the combustion chamber, which is not limited to the circular cylinder, is to be understood as including a polygonally-united tubular body. Also, a combustion means for burning a fuel in the combustion chamber may not be limited to specific one, but may include a burner and/or a combustion catalyst.
The reforming apparatus includes two modes relating to arrangement relation between the combustion chamber and the reforming reaction unit. One is a case where the reforming reaction unit is accommodated in the combustion chamber (FIGS. 23 to 27), another is a case where the reforming reaction unit is arranged around the combustion chamber in contact therewith (FIGS. 1 to 22). These two cases differ in the following point. In the former case, the reforming apparatus is just only heated from around thereof without heat dissipation from a surface thereof. While in the latter case, there is heat dissipation from around the reforming reaction unit.
Preferably an incombustible core is arranged at a center of the combustion chamber (FIGS. 9, 11, 14 to 17, 21, 22 and 24 to 26). That is, a flow space along which a combustion gas flows in the combustion chamber is narrowed by the core to increase a flow velocity of the combustion gas to thereby increase the efficiency of heat exchange with the reforming reaction unit. Preferably the core has a low heat capacity so that temperature rise of combustion chamber will not be hampered, and a hollow body is illustrated as an example of the core.
In the present invention, a method for indirectly heating the shift reaction unit and the CO oxidation unit includes: (1) a method of using heat conduction in solid or radiant heat conducted from the outer periphery of the combustion chamber through an intervening medium (FIGS. 1 to 7, and 27) and (2) a method of using heat of a burned exhaust gas flowing from the combustion chamber (FIGS. 8 to 27).
The reforming apparatus in which the shift reaction unit and the CO oxidation unit are indirectly heated by the method (1), may comprise the reforming reaction unit arranged around the combustion chamber in contact with an outer periphery of the combustion chamber and both of the shift reaction unit and the CO oxidation unit arranged around the reforming reaction unit.
In this reforming apparatus, the reforming reaction unit forms the intervening medium, and heat from the combustion chamber is transmitted to the shift reaction unit and the CO oxidation unit after having been decreased through the reforming reaction unit. Particularly, since the steam reformation performed at the reforming reaction unit is an endothermic reaction, heat from the combustion chamber decreases as a result of being consumed in the reforming reaction unit and thereafter conducts to the shift reaction unit and the CO oxidation unit. Also in this structure, since the reforming reaction unit, the shift reaction unit and the CO oxidation unit are arranged around the combustion chamber occupying the center of the reforming apparatus, it is effective for decreasing the height of the apparatus.
In addition, it is preferable that a partition wall having a function of regulating heat transfer is interposed between the reforming reaction unit and both of the shift reaction unit and the CO oxidation unit. Here, the above partition wall having a function of regulating heat transfer is intend to encompass any partition wall capable of regulating the temperature of heat to be transmitted down to the temperature range required by the shift reaction unit and the CO oxidation unit by decreasing the amount of heat transfer from the reforming reaction unit so that a residual heat of an unduly high temperature in the reforming reaction unit may be not transmitted directly to the shift reaction unit and the CO oxidation unit positioned outside thereof. This partition wall may comprise heat insulating materials, air layer or the like, and an optimum function of regulating heat transfer in the partition wall can be obtained by suitably selecting the kind of the material and thickness thereof. In this reforming apparatus, since the amount of heat transfer from the reforming reaction unit can be regulated by the partition wall, it is easy to control the temperature of the shift reaction unit and the CO oxidation unit.
In addition, a flow path connecting between the reforming reaction unit and the shift reaction unit may detour outside both of the shift reaction unit and the CO oxidation unit A reformed gas, immediately after emerging outwardly from the reforming reaction unit, is usually higher in temperature than the required temperature in the shift reaction unit, but the reformed gas can dissipate heat if the flow path connecting between the reforming reaction unit and the shift reaction unit detours outside of the shift reaction unit and the CO oxidation unit and, therefore, the reformed gas can be controlled to a suitable range of temperature.
In addition, it is preferable that the shift reaction unit is arranged on a side adjacent a high temperature zone of the reforming reaction unit and the CO oxidation unit is arranged on a side adjacent a low temperature zone of the reforming reaction unit, so as to be in conformity to a temperature distribution within the reforming reaction unit (see FIG. 2).
In the present invention, the reforming apparatus, in which the shift reaction unit and the CO oxidation unit are indirectly heated by the method (2), may comprise an exhaust chamber in which a burned exhaust gas from the combustion chamber directly flows, which the exhaust camber is positioned adjacent to and coaxially above the combustion chamber with the shift reaction unit arranged around the exhaust chamber and with the CO oxidation unit arranged around the shift reaction unit (see FIGS. 8 to 26).
In this reforming apparatus, the exhaust chamber is heated by a burned exhaust gas, the shift reaction unit is heated by heat transferred from around the exhaust chamber, and the CO oxidation unit is heated by heat transferred from the shift reaction unit. Since at this time, the temperature of the burned exhaust gas becomes lower than that of the combustion chamber, the shift reaction unit can be heated to lower temperature than that of the reforming reaction unit, and the CO oxidation unit placed outermost of the apparatus can be heated to lower temperature than that of the shift reaction unit. Therefore, the temperature of each reaction unit can be controlled to that required by respective reaction unit.
In this case, it is preferable to form a first air intake for introducing the fresh air in between the combustion chamber and the exhaust chamber (FIG. 12). Since the reformed gas immediately after having engaged outwardly from the combustion chamber, has a temperature as high as the combustion chamber, but the temperature of the shift reaction unit can be controlled by suitably cooling the burned exhaust gas with the fresh air from the air intake to reduce the temperature of the exhaust gas before the latter is fed to the exhaust chamber.
Additionally, in this case, it is referable to employ a secondary heating means for heating the exhaust chamber (FIGS. 16 and 17). The secondary heating means can be used for heating the exhaust chamber when the temperature to which the shift reaction unit is heated is low, and also for preheating the shift reaction unit at an early stage of preparation of the reformed gas.
Also, the reforming apparatus may be a construction that includes an exhaust vent for discharging the burned exhaust gas in the exhaust chamber to the outside, a shutter means for selectively opening and closing the exhaust vent, a first duct which is separated from the exhaust chamber and interposed between the shift reaction unit and the CO oxidation unit, and a second duct which is fluid-connected with the first duct and arranged around the CO oxidation unit (FIG. 21). In this reforming apparatus, a burned exhaust gas in the exhaust chamber flows to the first duct when the shutter means closes the exhaust vent and further flows to the second duct. At this time, the shift reaction unit and the CO oxidation unit are also heated by the burned exhaust gas then flowing through the first and second duct. On the other hand, when the exhaust vent is opened, the burned exhaust gas in the combustion chamber flows from the exhaust vent to outside and will not flow to the first and second duct. Therefore, the temperature of each of the shift reaction unit and the CO oxidation unit can be freely controlled by selectively opening or dosing the exhaust vent with the shutter means.
It is preferable to employ an air intake for introducing the fresh air into the second duct (FIG. 22). The use of this air intake makes it possible to cool only the burned exhaust gas then flowing through the second duct with the fresh air introduced into the second duct when the exhaust vent is closed by the shutter means, and therefore the CO oxidation unit can be more preferably controlled as to its temperature.
Also, it is preferable to employ an incombustible core in the center of the exhaust chamber (FIGS. 10, 11, 14 and 15). In this case, an effect similar to that brought about by the aforementioned core employed in the combustion chamber can be obtained at the exhaust chamber.
In the reforming apparatus according to the present invention, it is preferable that at least one of the reforming reaction unit, the shift reaction unit and the CO oxidation unit is provided on a surface thereof with a heat transfer material having a higher heat conductivity than that of a material forming the surface (FIG. 15). Each reaction unit has a tendency to develop a varying temperature along the direction of flow of the gas. For example, the reforming reaction unit has a tendency to exhibit a temperature drop on a leeward side thereof because of the endothermic reaction taking place therein, and the shift reaction unit and the CO oxidation unit have a tendency to exhibit a temperature rise on the respective leeward side thereof because of the exothermic reaction taking place therein. The heat transfer material provided on the surface of the reaction unit performs a role to level off the above difference in temperature.
Also, in the reforming apparatus according to the present invention, the CO oxidation unit may have an outer surface thereof provided with a fin for heat dissipation (FIGS. 19 and 20). In the event that the amount of heat transfer conducted from the heat source to the CO oxidation unit is excessive, heat can be dissipate from the fin to control the temperature of the CO oxidation unit so as to fall within the range of reactive temperature required by the CO oxidation unit.
Alternatively, the reforming apparatus in which the shift reaction unit and the CO oxidation unit are indirectly heated by the method (2), may have a construction that includes a main exhaust chamber in which a burned exhaust gas from the combustion chamber directly flows, a main exhaust vent for directly discharging the burned exhaust gas in the main exhaust chamber to the outside, a shutter means for selectively opening and closing the main exhaust vent, a first duct which is separated from the main exhaust chamber and fluid-connected thereto and is arranged around the main exhaust chamber, and a second duct which is fluid-connected with the first duct and arranged around the first duct, wherein the shift reaction unit is placed in the first duct and the CO oxidation unit is placed in the second duct (FIGS. 23 to 26).
In this reforming apparatus, when the main exhaust vent is closed by the shutter means, the burned exhaust gas from the combustion chamber flows through the first duct and then through the second duct, and the shift reaction unit placed in the first duct and the CO oxidation unit placed in the second duct are heated by the above burned exhaust gas. On the other hand, when the main exhaust vent is opened, the burned exhaust gas from the combustion chamber is discharged mainly from the main exhaust vent through the main exhaust chamber and will not flow to the first and second duct. At this time, the shift reaction unit and the CO reaction unit are heated by heat conduction in solid and radiant heat conducted from the main exhaust chamber, and also heat of burned exhaust gas flowing inside thereof. Thus, during an early running of this reforming apparatus before preparation of the reformed gas, not only the reforming reaction unit but also the shift reaction unit and the CO oxidation unit can be preheated by previously burning in the combustion chamber while the main exhaust vent is dosed by the shutter means. During a steady running of this reforming apparatus, each reaction unit can be more preferably controlled as to its temperature by keeping the main exhaust vent in an opened position.
This reforming apparatus can include an exhaust sub-vent for discharging a burned exhaust gas within the first duct to the outside, and a shutter means for selectively opening and closing the exhaust sub-vent (FIGS. 21 to 26). In this case, when the exhaust sub-vent is kept open while the main exhaust vent is opened, a burned exhaust gas slightly flows to the first duct from a separating portion between the main exhaust chamber and the first duct, and then discharged from the exhaust sub-vent. The burned exhaust gas slightly flowing through the first duct works to heat the shift reaction unit to a somewhat higher temperature range than that in the CO oxidation unit. Therefore, each reaction unit can be more preferably controlled as to its temperature.
In addition, it is preferable that at least one of the shift reaction unit and the CO oxidation unit is formed into a coil-like shape (FIGS. 23 to 26). In this case, because the shift reaction unit and the CO oxidation unit is formed into a coil-like shape, efficiency of heat exchange becomes good when the shift reaction unit and the CO oxidation are heated by a burned exhaust gas.
Also, it is preferable to form an air feed channel for introducing the fresh air into the second duct (FIGS. 23 to 26). In this case, the CO oxidation unit can be temperature-controlled by the fresh air introduced from the air feed channel into the second duct into.
In the reforming apparatus according to the present invention, it is preferable that at least a portion of a raw material feed channel for feeding the raw material and steam to the raw material reforming unit is arranged in a position in which the raw material and the steam are preheated by heat from the heat source of the raw material reforming unit (FIGS. 3 to 27).
That is, while the raw material reforming unit is fed with the raw material and steam which are in a state of mixture through the raw material feed channel, the capability of the raw material feed channel being preheated facilitates generation of steam from water in the raw material feed channel and, therefore, water rather than steam can be supplied from a source of the raw material to the raw material feed channel. This dispenses with necessity of use of a separate steam generating apparatus and, consequently, a reforming system can be downscaled as a whole. Also, since the preheating of the raw material feed channel allows the raw material and steam to be heated to a temperature close to the temperature range required for the steam reformation, the reformation reaction in the raw material reforming unit can be immediately initiated in an early state of the raw material reforming unit without the temperature of a reformation catalyst therein being lowered.
Though the method for preheating the raw material feed channel is not limited to specified one, another preheating method may be employed in which, for example, at least a portion of the raw material feed channel is held in contact with the surface of at least one of the reforming reaction unit, the shift reaction unit and the CO oxidation unit (FIGS. 3 to 6, 8 to 24, and 26); at least a portion of the raw material feed channel is arranged at a position contactable with the burned exhaust gas from the heat source of the raw material reforming unit (FIG. 7); or at least a portion of the raw material feed channel is arranged at such a position that it can be directly heated by the heat source of the raw material reforming unit (FIGS. 25 and 27).
Also, in the reforming apparatus according to the present invention, where the heat source of the raw material reforming unit generates heat by catalytic combustion, it is preferable to employ a preheating means for preheating the combustion catalyst held in the heat source (FIG. 18). In the case of the heat source generating heat by catalytic combustion, the combustion do not start until when temperature of the combustion catalyst rises up to some degree but, the combustion reaction can start immediately in early time of beginning combustion if the combustion catalyst is preheated by the preheating means in advance.
The reforming apparatus according to the present invention is particularly effective where the raw material to be reformed is employed of a kind which the reactive temperature of reformation reaction thereof is in a high temperature range. For example, in the case of butane used as the raw material, it is necessary that the reforming reaction unit is heated to the range of 400 to 1000xc2x0 C.; the shift reaction unit is heated to the range of 200 to 350xc2x0 C.; and the CO oxidation unit is heated to the range of 100 to 250xc2x0 C. When the reactive temperature of the reforming reaction unit is in the high temperature range like this case, a difference between reactive temperature required in the reforming reaction unit and that required in the shift reaction unit and the CO oxidation unit becomes so large that the temperature control thereof becomes difficult to accomplish. However, in the reforming apparatus according to the present invention, as above mentioned, since the three reaction unit is independent from each other and not only is the reforming reaction unit directly heated from the combustion unit, but the shift reaction unit and the CO oxidation unit are indirectly heated by heat transfer from the combustion unit, the temperature of the reforming reaction unit can be easily controlled to the high temperature range, and that of the shift reaction unit and the CO oxidation unit can also be easily controlled to the low temperature range.